Brian C. Welch

Spatial variation of radar-derived basal conditions on Kamb Ice Stream, West Antarctica

Abstract Radar profiles of bed echo intensity can survey conditions at the ice–bed interface and test for the presence or absence of water. However, extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice. We used the relationship between bed echo intensities from constant-offset radar data and ice thickness to estimate depth-averaged attenuation rates at several locations on and near Kamb Ice Stream (KIS), West Antarctica. We found values varying from 29 dBkm–1 at Siple Dome to 15 dBkm–1 in the main trunk region of KIS, in agreement with a previous measurement and models. Using these attenuation-rate values, we calculated the relative bed reflectivity throughout our KIS surveys and found that most of the bed in the trunk has high basal reflectivities, similar to those obtained in the location of boreholes that found water at the bed. Areas of lower bed reflectivity are limited to the sticky spot, where a borehole found a dry bed, and along the margins of KIS. These results support previous hypotheses that the basal conditions at locations like the sticky spot on KIS control its stagnation and possible reactivation.

The International Trans-Antarctic Scientific Expedition (ITASE): an overview

Abstract From its original formulation in 1990 the International Trans-Antarctic Scientific Expedition (ITASE) has had as its primary aim the collection and interpretation of a continent-wide array of environmental parameters assembled through the coordinated efforts of scientists from several nations. ITASE offers the ground-based opportunities of traditional-style traverse travel coupled with the modern technology of GPS, crevasse detecting radar, satellite communications and multidisciplinary research. By operating predominantly in the mode of an oversnow traverse, ITASE offers scientists the opportunity to experience the dynamic range of the Antarctic environment. ITASE also offers an important interactive venue for research similar to that afforded by oceanographic research vessels and large polar field camps, without the cost of the former or the lack of mobility of the latter. More importantly, the combination of disciplines represented by ITASE provides a unique, multidimensional (space and time) view of the ice sheet and its history. ITASE has now collected >20 000km of snow radar, recovered more than 240 firn/ice cores (total length 7000 m), remotely penetrated to ~4000m into the ice sheet, and sampled the atmosphere to heights of >20 km.

Mapping subglacial surfaces of temperate valley glaciers by two-pass migration of a radio-echo sounding survey

High-resolution maps of the glacier bed are developed through a pseudo-three-dimensional migration of dense array of radio-echo sounding profiles. Resolution of three-dimensional maps of subglacial surfaces is determined by the radio-echo sounding wavelength, data spacing in the field, and migration. Based on synthetic radio-echo sounding profile experiments, the maximum resolution of the final map cannot exceed one half-wavelength. A methodology of field and processing techniques is outlined to develop a maximum-resolution map of the glacier bed. The field and processing techniques are used to develop a map of the glacier bed below part of Worthington Glacier, a temperate valley glacier in south-central Alaska. The field techniques and the processing steps used on the glacier result in a map of 20 m × 20 m resolution.

First results from radar profiles collected along the US-ITASE traverse from Taylor Dome to South Pole (2006-2008)

Abstract The 2006/07 and 2007/08 US-ITASE traverses from Taylor Dome to South Pole in East Antarctica provided opportunities to survey the subglacial and englacial environments using 3 MHz and 200MHz radar. We present first results of these new ground-based radar data. A prominent basal deformation layer indicates different ice-flow regimes for the northern and southern halves of the Byrd Glacier drainage. Buried dune stratigraphy that appears to be related to the megadunes towards the west occurs at depths of up to 1500 m. At least two new water-filled subglacial lakes were discovered, while two recently drained lakes identified from repeat ICESat surface elevation surveys appear to be devoid of water.

A time marker at 17.5 kyr BP detected throughout West Antarctica

Abstract Deep radar soundings as part of the International Trans-Antarctic Scientific Expedition (US-ITASE) traverses in West Antarctica have revealed a bright internal reflector that we have imaged throughout widespread locations across the ice sheet. The layer is seen in traverses emanating from Byrd Station in four directions and has been traced continuously for distances of 535km toward the Weddell Sea drainage, 500km toward South Pole, 150km toward the Executive Committee Range and 160km toward Kamb Ice Stream (former Ice Stream C). The approximate area encompassed by the layer identified in these studies is 250 000km2. If the layer identification can also be extended to Siple Dome where we have additional radar soundings (Jacobel and others, 2000), the approximate area covered would increase by 50%. In many locations echo strength from the layer rivals the bed echo in amplitude even though it generally lies at a depth greater than half the ice thickness. At Byrd Station, where the layer depth is 1260 m, an age of ~17.5 kyr BP has been assigned based on the Blunier and Brook (2001) chronology. Hammer and others (1997) note that the acidity at this depth is >20 times the amplitude of any other part of the core. The depiction of this strong and widespread dated isochrone provides a unique time marker for much of the ice in West Antarctica. We apply a layer-tracing technique to infer the depth–time scale at the inland West Antarctic ice sheet divide and use this in a simple model to estimate the average accumulation rate.

Spatial variability in the flow of a valley glacier: Deformation of a large array of boreholes

Measurements of the deformation of a dense array of boreholes in Worthington Glacier, Alaska, show that the glacier moves with generally bed-parallel motion. Strain in the 200 m deep valley glacier is constant near the surface but follows a nonlinear vertical profile below a depth of about 120 m. By a depth of 180 m, the octahedral strain rate reaches 0.35 yr -1 . The three-dimensional velocity field shows spatial complexity with significant deviations from plane strain, despite relatively simple valley geometry in the vicinity of the 6 × 10 6 m 3 study volume. No evidence was found for time-varying deformation or movement along localized shear planes. Observations were made by repeatedly measuring the long-axis geometry of 31 closely spaced boreholes over a 70 day period, and three additional holes after 1 full year of deformation. The holes were spaced 15 to 30 m apart. Installation and measurement of such a large number of boreholes required the development of a semiautomated hot water drilling system that creates straight and vertical boreholes with uniform walls. The equipment and procedures enables borehole profiles to be measured without the use of hole casing. Inclinometry measurements collected in the holes were processed, analyzed for error, and visualized as a fully three-dimensional data set. The new methods offer unique insight into small-scale spatial and temporal variations in the pattern of flow in a valley glacier.

Bedrock topography and wind erosion sites in East Antarctica: observations from the 2002 US-ITASE traverse

Abstract Ice stratigraphy from deep-penetrating radar data collected during the 2002 US International Trans-Antarctic Scientific Expedition (US-ITASE) traverse shows evidence of a significant erosion surface and drift-filled basin related to a previously undiscovered 1400m subglacial mountain between Hercules Dome (87˚420 S, 108˚ W) and South Pole. The 3MHz radar profile crosses three subglacial mountains at approximately 458 to the ice-flow direction. Cross-cutting reflectors in the top 500m of ice stratigraphy are interpreted as angular unconformities resulting from wind erosion as the ice deforms over the mountain tops. The unconformities correlate locally with zones of high RADARSAT reflectivity. Several nearby sites with similar relatively high RADARSAT reflectivity adjacent to the traverse indicate that active wind erosion may be taking place at these locations as well. Based on the local correlation between surface wind scour and subglacial topography, we interpret the nearby cluster of bright RADARSAT reflectivity to indicate the presence of a small range of subglacial mountains. The ability to trace isochronal stratigraphy, associated with scour sites using shallow and deep radar, to nearby dated ice cores presents the possibility of exploiting wind-scour zones to access well-dated older ice with shallow-coring equipment.

The evolution of surface flow stripes and stratigraphic folds within Kamb Ice Stream : why don't they match?

Flow stripes seen in satellite imagery of ice streams and ice shelves are caused by surface undulations with kilometer-scale spacing and meter-scale relief and generally indicate current or recent fast ice flow. On a similar scale, folding of internal ice stratigraphy depicted in cross-flow ice- penetrating radar profiles is also a common occurrence in ice streams, suggesting a possible relationship between the two sets of features. We have traced surface flow stripes in RADARSAT and MODIS imagery on Kamb Ice Stream, West Antarctica, from the onset of streaming flow into the near-stagnant trunk. We compare the morphology and evolution of the surface flow stripes to the folds seen in the internal stratigraphy in cross-ice-stream radar profiles. We find essentially no correspondence in the observed locations or spacings between the radar internal layer folds at depths greater than 100 m and the flow stripes on the surface. The gap in the radar data and the surface mappings in the top 100 m of firn prevents a precise depiction of how the flow stripes and fold patterns at depth diverge. We explore hypotheses about how flow stripes and internal stratigraphic folds can originate and evolve differently as ice flows downstream. We suggest that flow stripes are subject to surface processes that can modify their morphology independently of the internal stratigraphy, leading to changes in the pattern of flow stripes relative to the internal layers below.

Geophysical survey of the intra-caldera icefield of Mt Veniaminof, Alaska

Abstract Mt Veniaminof is a large active stratovolcano located on the Alaska Peninsula (56.2° N, 159° W). We present results of the first geophysical survey of the icefield that fills much of the 10 km×8 km caldera that was most recently modified during the last major eruption roughly 3700 BP. The subglacial topography and ice volume are derived from an 8MHz radio-echo sounding survey conducted in July 2005. Prominent internal reflectors are assumed to be isochronal ash/acid deposits related to local eruptions. Accumulation rates and basal melt rates are calculated using a Nye one-dimensional steady-state accumulation model applied at a location that approximates an ice divide and calibrated by matching internal reflectors with published records of recent local volcanic eruptions. The model yields order of magnitude estimates of the accumulation rate of 4ma–1 water equivalent and 2 ma–1 of basal melt. The subsequent geothermal flux of ∽19Wm–2 is similar to active hydrothermal vents in volcanic lakes. We suggest that these values represent an upper limit for the geothermal flux within the ice-covered regions of the main caldera. We also analyze likely subglacial meltwater flow paths to examine the implications of recent eruption activity at an active intra-caldera cinder cone. Two lava-producing eruptions from the cinder cone in 1983–84 and 1993–94 melted roughly 0.17km3 of ice. The lack of significant deformation of the internal stratigraphy to the south and east of the melt hole suggests that any subglacial drainage in those directions was entirely within subglacial deposits. We suggest that the more likely drainage route was northwest into a large outlet glacier.

Persistent Tracers of Historic Ice Flow in Glacial Stratigraphy near Kamb Ice Stream, West Antarctica

This archive includes a pdf containing a data description, a location map, and quick-look images of the data, as well as 11 .mat (MATLAB) binary files containing the radar data.